Gas turbine components, such as superalloy blades and vanes, are subjected to high temperatures and stresses during engine operation. Under such conditions they will often become physically damaged due to the formation of cracks, voids and worn surfaces. When the damage extends beyond certain allowable limits, a decision must be made to either repair or replace the components. Because they are expensive to manufacture, there is considerable economic incentive to attempt repair of turbine components by methods such as welding, brazing or wide-gap brazing.
Wide-gap brazing refers to the repair of defects too large to be filled or bridged by standard brazing techniques wherein the gap filler material is drawn into defects by capillary forces alone. Therefore, wide-gap filler materials must to be physically pre-placed within joints or defects or onto surfaces and, during heat treatment, exhibit sluggish flow which prevents them from substantially flowing out of the repair area. Prior art wide-gap filler compositions are typically comprised of a mixture of superalloy and braze alloy powders suspended in some type of temporary organic vehicle so as to form a slurry, paste or transfer tape. The organic vehicle, or binder, is usually comprised of an organic polymer dissolved in a solvent, and sometimes includes a plasticizer and dispersant. The organic polymer provides strength to the alloy powder deposit after the solvent has evaporated, bonding the powder particles to each other, as well as to the substrate article. (The word “binder” is usually used to mean all of the ingredients present in the vehicle, including the organic polymer, solvent, plasticizer, wetting agent, etc. However, in many references, the word “binder” refers specifically to the organic polymer constituent of the vehicle. When used herein, the word “binder” shall be used in the traditional sense and the phrase “principle binder resin” shall be used to refer to the organic polymer component.) Subsequent drying and furnace heat treatment operations decompose and vaporize the various binder constituents, followed by brazing or sintering of the powder. Alloy powders used for wide-gap filler materials are described in U.S. Pat. Nos. 4,073,639 and 4,381,944. Organic binders and methods used in the formulation of slurries, pastes and transfer tapes have been described in U.S. Pat. Nos. 2,908,072; 3,293,072 and 3,589,952.
Historically, wide-gap repairs were developed for the repair of defects in aero or aeroderivative gas turbine components. Relatively speaking, these components and the defects in them tend to be small. For example, a typical wide-gap crack in an aero gas turbine component might be about ¼ inch in length by about 0.030 inches in width or depth. In contrast, heavy frame gas turbines which are designed primarily for industrial power generation are much larger than aero or aeroderivative gas turbines. A single vane segment or blade from one of these engines can weigh upwards of 100 lbs. Crack defects in these components are correspondingly much larger, with dimensions often exceeding several inches in length and up to one inch in width or depth. Standard welding techniques cannot always be used to successfully repair this type of damage, and it is again desirable to be able to use some type of wide-gap repair process for component restoration.
While the wide-gap slurries, pastes and transfer tapes of the prior art have been found useful for the repair or joining of the smaller areo components, there are many situations in which these materials are unsatisfactory for the repair of larger defects in heavy frame gas turbine components. For instance, it is often desirable to be able to apply the wide-gap filler material to thicknesses of ⅛ inch or more onto surfaces with vertical or inverted orientations. After it is applied, the filler material should neither flow, shrink, nor form defects such as voids, tears and the like during subsequent handling and heat treatments. Prior art wide-gap repair materials will either slump or fall off the article during drying and/or heat treatment when used in this way, making it necessary to complete the brazing or liquid phase sintering operation in a number of steps by varying the orientation of the article in the furnace each time.
Some additional requirements of a good wide-gap filler material are that, during its initial application, it should be capable of plastic flow together with adhesive properties which are similar to those of a modelling clay. These properties would allow the alloy powder mixture to flow into a desired shape by applying a moderate force, for example by hand, and thereafter keep its shape, while adhering to the substrate article in various orientations. Once the external force is removed from the wide-gap filler, it should keep its shape while the repair article is handled, stored, dried, and heat treated. These attributes are not found in the wide-gap slurries, pastes and transfer tapes of the prior art. For example, a powder metallurgy repair material, comprised of a mixture of iron-base alloy powders and a plastic binder, has been described in connection with the repair of centrifugal pump impellers (Welding Journal, April 1971, pp. 255–256). The proprietary materials used in this method were molded by hand, however, back-up supports were needed on the underside of through-going defects to hold the powder mix in place. In other words, the mixture was not self-supporting.
Still another limitation of prior art wide-gap filler materials has been encountered in the repair of hollow gas turbine components which contain through-going defects or details. In most cases, drop-through or flow of the repair filler material into interior cooling passages or cavities cannot be tolerated, since obstruction of these passages would render the component unserviceable and unrepairable due to the limited access afforded by the component design. It is very difficult to control the flow of prior art wide-gap pastes and slurries which makes them unsuited to the repair of these types of defects or details. An important advantage of the wide-gap filler material of the present invention is that the aforementioned limitations related to molding, flow, slumping and loss of adhesion can be eliminated. This advantage is realized through the use of the novel sacrificial binder system of the present invention.
Finally, within the general category of materials comprised of metal powder alloys and organic binders there exists another class of materials which are used in the powder injection molding art. Powder injection molding (herein referred to as PIM) is a method for the fabrication of ceramic or metallic sintered parts. A solid green body or compact comprised of a ceramic or metallic particulate material and a sacrificial binder mixture is molded in a die by the application of heat and mechanical pressure in an injection molding machine. The binder ingredients are later removed from the green body in a series of solvent or thermal debinding processes, followed by firing and sintering of the compact. The main PIM binder types are thermoplastic, thermosetting and gelation systems (R. M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, N.J., 1990, pp. 99–124). Thermoplastic sacrificial binders used in the formulation of PIM feedstock are rigid and non-adhesive at room temperature and must be softened by heating before the mixture will flow adequately to allow mold filling. Thermosetting and water-based gelation binders develop their strength by cross-linking of the polymer units at elevated temperatures. Rigid, self-supporting compacts can only be produced from these materials by heating the die cavity after the feedstock has been introduced. The need for substantial temperature and pressure variations during the processing of PIM feedstocks makes the binders used in these formulations unsuitable for use in conjunction with the wide-gap filler materials of the present invention.